Steam power generating apparatus



June 26, 1962 s. M. ARNOW 3,040,537

STEAM POWER GENERATING APPARATUS Filed Sept. 28, 1960 5 Km J m w N 5 m1?1% d m M 5V.

United States Patent 3,040,537 STEAM PGWER GENERATING APPARATUS SamuelM. Arnow, Philadelphia, Pa., assignor, by mesne assignments, toBaldwin-Lima-Hamilton Corporation, Philadelphia, Pa., a corporation ofPennsylvania Filed Sept. 28, 1960, Ser. No. 59,078 6 Claims. (Cl. 60-67)The present invention is directed to a modification of the steam powergenerating apparatus described and claimed in US. Patent 2,883,832granted to me April 28, 1959, on an application filed May 3, 1956.

The steam power generating apparatus as described and claimed in myaforesaid patent is very efiicient when employed in a steam powergenerating plant which is operating at full load or at substantiallyfull load. When, however, the plant is operating at less than full load,for example, one-half load, my previous invention becomes ineffective.The present invention is directed to means in the heat cycle forrestoring effectiveness during periods of such reduced load and toapparatus for activating such means automatically during operation atthe reduced load.

This invention relates to steam power generating apparatus, and moreparticularly to a steam power generating station in which turbines areutilized to drive electric generators, for example. In such plants theheat carried away by the combustion gases from the boilers representsone of the chief sources of efficiency loss.

It is an object of this invention to provide a steam power generatingapparatus including an efficient means for tempering incoming air.

Another object is to provide an efiicient steam power generatingapparatus arranged for absorbing heat from exhaust gases, for preheatingcondensate, and for tempering incoming air, all in combination.

Still another object of this invention is to provide a steam boilerhaving an air preheater and which includes a readily controllableincoming air tempering means for introducing air at a substantiallyconstant temperature to the preheater, notwithstanding fluctuations oftemperature of the air introduced into the tempering means.

Another object is to provide an efiicient means for utilizing hotexhaust gases for preheating the condensate at the most effective pointin the steam power thermal cycle, without condensing ingredients of theexhaust gases or corroding the preheater.

It is a further purpose of the present invention to accomplish all ofthe foregoing objects irrespective of whether the steam generating plantis operating at full load or at substantially less than full load.

Still a further object of this invention is to provide means forfeedwater heating and for air pre-tempering from exhaust steam withdrawnfrom turbine bleeds at different pressure bleed points according toloading.

Other objects and advantages of this invention, including the simplicityand economy of the same, and the ease with which it may be applied toexisting steam power generating systems, will further become apparenthereinafter and in the drawing.

Referring to the drawing, the number 1 represents generally a boilerhaving a gas exit 2 connected through breeching 3 to a stack 4, thedraft being induced by a blower or blowers designated 5.Combustion-supporting air is introduced into the lower portion of thefire box of the boiler 1 by way of a duct 6 by action of a blower orblowers 7, said duct passing through the gas exit 2 of the boiler andhaving there interposed in it a preheater 3,040,537 Pitented June 26, 1962 8 which absorbs some of the flue gas heat, whereby the line 31.

air is preheated before entering the boiler.

Steam is conducted from the boiler 1 through a'conventional pipe 9 todrive a turbine 10 having an associated condenser 11. Leading fromdifierent bleed points of the turbine 10 are pipes 12, 13, 14A and 14Bthrough which exhaust steam at different pressures is channeled tofeedwater heaters, with pipes 12 and 13 feeding feedWa-ter heaters 15and 16 respectively and with pipes 14A and 14B leading into a commonpipe 14 which feeds feedwater'heater 17. Pipe 14B (and also, if desired,pipe 14A) is provided with a check valve 44 (and 43) for preventing flowof steam exhausted through pipe 14A from flowing back to the turbine 10through the pipe 14B. Pipes 14A and 14B are each also provided with acontrol valve 45, 46 respectively whose function and manner of operationwill be described hereinafter. It will merely be mentioned at this pointthat for greatest economy of operation the feedwater heater 17 should beconnected to that bleed point nearest to the low pressure exhaust of theturbine at which the pressure and temperature of the exhaust steam arenevertheless sufiiciently high to avoid causing the stack temperature tobe pulled down to a temperature sufficiently low to cause or to tend tocause condensation and corrosion.

The condensate from the condenser 11 is propelled, by a plurality ofpumps diagrammatically shown as a pump 18, through a feed pipe line 19which extends to a coil 20 in the feedwater heater 17, then, inaccordance with my invention described in US. Patent 2,883,832, to a lowlevel economizer 21 in the breeching 3. It should be understood that thecoil 20, although conventionally referred to and illustrated as a coil,in practice is actually a system of tubes. It is further to beunderstood that a system of tubes well known in the art as used for heattransfer purposes, will hereinafter be referred to for convenience as acoil. The feedwater pressure in economizer 21 should be aboveatmospheric, to prevent leakage of exhaust gases into the feed water.Continuing on, the line 19 extends first to a coil 22 in the feedwaterheater 16, then to a coil 23 in a feedwater heater 15, through aplurality of boiler feed pumps diagrammatically shown as a pump 25. Fromthe pumps 25, line 19 extends through an econornizer 26 (which latter islocated within the boiler), and finally to the boiler 1 in which theline 19 terminates. An additional feedwater heater 27, or a group ofthem, may also be provided, respectively fed by ascending pressure bleedpoints.

Provided in the air duct 6 is a water-to-air heat exchanger 30 which isset into the air inlet duct 6 and which is interposed in a closed watercirculation pipe Pipe line 31 also includes a coil 32 located within thefeedwater heater 17. The water in the line 31 is under positive pressureand circulated by a pump indicated at 33, such water being purified sothat any possible leakage into the heater 17 would not contaminate thecondensate. Also, since the water in line 31 is under positive pressure,there is no possibility of leakage of air into the line '31 at the airtempering heater 30 to contaminate the water. A conventional cascadingsystem of heater condensate drains extends from heaters 27, 15, 16 and17 back to condenser 11. This is a closed system sealed from theatmosphere, thus preventing air from entering heater 17. The rate ofcirculation is requested by a temperature-responsive valve 34 controlledby a thermostat 35 having its sensing element located within the airduct portion 36, which is downstream relative to the Water-to-air heater30. Valve 34 is downstream relative to the pump 33.

It is important to observe that the feedwater heater 17 is connected totransfer heat through both the coils 32 and 21). Working atsub-atmospheric pressure, at the bleed point of lowest possible pressureconsistent with stack temperature, the heater 17 is ideally suitedeconomically for pre-heating the Condensate prior to its introductioninto the low level economizer 21, because the work done by the steam inthe turbine is at a maximum when bled off at this low pressure.Moreover, heat is concurrently transferred by heater 17 to the water inpipe 31, whereby both the low level economizer 21 and the temperingheater 30 are heated from a common, sub-atmospheric pressure source.

In operation, the temperature of the gases as they leave the airpreheater of the conventional steam generating system is usuallymaintained at about 260 F and the air entering the preheater isdesirably not less than about 100 F.; otherwise during low outsidetemperature periods the flue gases leaving the preheated may be cooledbelow the dew-point and the maintenance cost of the pre-heater wouldbecome excessive because of corrosion resulting from condensation. Inorder to avoid this difliculty, it is conventional to temper the air byincreasing its temperature by about 60 F. before it enters thepre-heater. For 65 F. (annual average) incoming air, the temperaturewould be raised to about 125 F., which is safely above the desiredminimum. Steam is used for this purpose and is usually withdrawn fromone of the bleed points in the turbine. However, using such a method itwas not practical to control the extent of preheating. Prior to myinvention, as described in US. Patent 2,883,832, in order to safeguardagainst contamination of the steam by the air, the bleed point selectedmust have had a pressure above atmospheric pressure at all operatingloads.

In accordance with the improved air pre-heating system of my aforesaidPatent 2,883,832, a significant gain in economy is achieved bywithdrawingthe steam at the lowest pressure bleed point, which is belowatmospheric pressure and in one practical case, for example, has a fullload pressure of approximately only 5.7 pounds per square inch absolute.Notwithstanding the fact that the steam is withdrawn at a pressure belowatmospheric, steam contamination is nevertheless avoided. As was beforeindicated, the withdrawal of the steam at such low pressure bleed pointresults in an increase in work done by the turbine before the steam iswithdrawn.

Also, in accordance with my aforesaid Patent, 2,883,- 832, an evengreater gain in economy is achieved by causing the feedwater heatingcircuit to pick up heat from the flue gases ahead of the stack. Thisheat, which includes the heat added by the induced draft fans would, ina prior art plant, have been lost up the stack. By employing the fluegases to assist in heating 'the feedwater circuit, I am able in myPatent 2,883,832 to dispense with the bleed point 40 of the turbinewhich otherwise would have been connected to an additional feed- Waterheater interposed between the feedwater heaters 1 6 and 17. By normallynot using the bleed 40 located intermediate of bleed points 13 and 14Bincreased work is done in turbine 10 by the steam which otherwise wouldhave been extracted from the bleed point 40. In the present example,bleed point 40 is located at a point at which the pressure is about 14pounds per square inch absolute (p.s.i.a.) at full loadwhereas the bleedlines 12 and 13 under normal full-load operating conditions are locatedat points at which the pressure is of the order of 54 and 22 pounds persquare inch absolute (p.s.i.a.), respectively, as i indicated in thedrawing.

The system described in my Patent 2,883,832 and also described thus farin my present patent application, achieves good economy of operation andfunctions very satisfactorily, provided the steam generating plant isoperating at or substantially at full load.

To bring out the modification introduced by my pres- 4 cut invention,assume that a plant which has been operating at full load under thepressure and temperature conditions mentioned above and shown in thedrawing reduces its operation to substantially less than full load, sayone-half load When this occurs, the pressure at the lowest bleed pointto which pipe 14B is connected is reduced from 5.7 p.s.i.a to about 2.8p.s.i.a., and the temperature of the steam pipe 14B drops from 167 F. toabout 139 F. This causes the water in pipe 19 leading up to theeconomizer 21 to drop from 164 F. to about 136 F. As a result, the 275F. gas in breaching 3 in passing economizer 21 drops to a temperaturebelow 200 F. and the temperature in stack 4 becomes say P. which is toolow to avoid the possibility of condensation and corrosion.

In accordance with my present invention, the drop in temperature instack 4 is sensed by a temperaturesensitive thermostat 60 having a pairof upper contacts and a pair 'of lower contacts and a bridging contactoror switch arm 61 so adjusted that when the stack temperature drops belowa selected point, say F., the switch arm 61 of the thermostat, which hadprior thereto been in the upper position, drops to the lower position,as shown in the drawing. This connects the lower contacts and opens theupper contacts of the switch. As a result, an electrical circuit frompower lines L1, L-2 which had been previously closed through amotor-operated valve 46 in pipe 143 is now open, and a previously opencircuit is now closed through the motoroperated valve 45 in pipe 14A.Motor-operated valves 45 and 46 may be assumed to be biassed, as by aspring or other means, normally to closed position and to be opened bymotor when the electrical circuit therethrough is closed. Thus, when theswitch arm 61 drops to the lower position to close the circuit throughthe motor valve 45, the valve 45 opens, while valve 46 whose motor is nolonger energized now closes. This connects bleed point 40 to thefeedwater heater 17 by Way of pipes 14A and 14, and shuts off pipe 14B.The pressure at bleed point 40, as previously indicated, is of order of14 p.s.i.a. at full load at a temperature of about 210 F. At half loadoperation, the pressure at bleed point 40 is of the order of 7 p.s.i.a.at about 177 F.

It will be seen then that when the temperature in stack 4 drops, thetemperature-sensitive thermostat 60 is eifective to switch the bleedconnection from the lowest bleed point on turbine 10 to the next higherbleed point. By so doing, the water temperature in pipe 19 leading tothe economizer 21 is raised again to the vicinity of 164 F. and thestack temperature is raised to 200 F. or higher. In this manner, thestack temperature is maintained well above the dew point, andcondensation and corrosion are thereby avoided irrespective of whetherthe steam generating plant is operating at full load or half load.

In order to provide for satisfactory and efiicient operation at lessthan full load, as well as at full load, the Water seal shown in myaforesaid Patent 2,883,832 is replaced in the apparatus of the presentpatent application with a float-operated valve 62 which allows water topass therethrough from right to left as viewed in the drawing but whichdoes not permit the passage of steam therethrough from left to right.The change from a water seal, as shown in my Patent 2,883,832, to afloatoperated valve 62 is necessary because of the impractical length ofwater seal which would be required for a heater connected to a 14p.s.i.a. bleed point.

For the purpose of indicating when the exhaust steam fed to heater 17 isbeing supplied from bleed point 40, a signal lamp may be connectedacross the terminals of water-valve 45.

In lieu of an automatic system for switching the bleed point from thelowestmost line 14B to the next higher line 14A, as just describedabove, the switchover may of course be made manually in response to thesounding of an alarm. The numeral 63 in the drawing represents such analarm which is set off when the switch arm 61 of thermostat 60 changespositoin, either from the upper to the lower position or from the lowerto the upper posi tion. The connections to valves 45 and 46 would bedisconnected since these valves would now be operated manually. V

In the drawing, alarm 63 is represented as having two switches 64 and65, which close electrically in response to current flow through thecoil associated therewith. When closed, the switches remain closed untilopened manually, as to shut oif the alarm. These switch elements are ofcourse, not necessary when the operation is fully automatic, as firstdescribed above. In manualalarm operation, with the stack theremostatswitch arm 61 in the upper position, switch 65 is in closed position(having been closed by the current through its coil before switch 64 isopened) and switch 64 is in open position, having been opened manuallyto shut off the alarm. When due to a drop in stack temperature, switcharm 61 drops to the lower position, current flows through lead 67, thealarm is actuated, and switch 64 is closed electrically by the currentflowing through its coil. The alarm continues to ring until it is shutoif manually by opening switch 65. Switch 64 remains closed. When, dueto a rise in stack temperature, the thermostat switch arm 61 rises tothe upper position, current flow through lead 66, the alarm is againactuated, and switch 65 is closed electrically by the current flowingthrough its coil. As before, the alarm continues to ring until shut offby manually opening switch 64, switch 65 remaining closed.

It will be understood, of course, that for manual operation of valves 45and 46, employing the operating alarm means 63 as described above, andalso for the fully automatic operation first described, the stackthermostat 60 would be so set that the switch arm 61 would not move intocontact with the upper contacts until the stack temperature had risen toa substantially higher temperature than that reached under say one-halfload operation, so that the connection from the turbine to the feedwaterheater 17 can be safely switched to the lowest pressure connection 14B.

The invention of the present application constitutes an importantimprovement to the basic system described and claimed in my basic Patent2,883,832, in that it permits a steam generating plant employing mybasic invention to be operated at less than full load, as well as atfull load. This is unquestionably an important advantage, as many steamgenerating plants are at times operated at less than full load and atother times are operated at full load.

The present invention senses automatically that the load has appreciablychanged by detecting the appreciable change in stack temperature. And,in the fully automatic form of operation, the apparatus provided by thepresent invention then automatically makes the necessary switch orchange in steam supply, switching from one bleed point to another andthereby simultaneously effecting the required change in steam supply tothe feedwater heater which controls both the stack econo-mizer and theair tempering heater.

While the preferred embodiment of this invention has been described insome detail, it will be obvious to one skilled in the art that variousmodifications may be made without departing from the invention ashereinafter claimed.

Having thus described my invention, I claim:

1. In a steam power plant; a boiler, including conducting means forexhausting combustion products, said conducting means terminatingoutwardly in a stack; a turbine driven by steam from said boiler; acondenser connected to condense steam from said turbine; a plurality ofsteam bleed points each at a different pressure point on said turbine,at least one of said pressure points being below atmospheric pressure atfull load; a plurality of feedwater heaters having water coils therein;means connecting different bleed points to different feedwater heaters,the bleed point of lowest pressure being connected to a first feedwaterheater, and other bleed points of increasingly higher pressure beingconnected to second, third, etc., feed-water heaters in that order; alow level economizer located in the path of flow of said combustion products; a feedwater pipe line extending from the condenser to a firstwater coil in said first feedwater heater, then to the economizer, thenthrough the coils of said second, third, etc., feedwater heaters insequence in that order, and then to the boiler; temperature-sensitiveswitch means positioned to be sensitive to the temperature in saidstack; a first motor-operated valve in the connection from the bleedpoint of lowest pressure in said turbine to said first feedwater heater;a second motor-operated valve in the connection from the bleed point ofnext higher pressure in said turbine to said first feedwater heater,said first and second motor-operated valves being connected electricallyto, and operative automatically in response to actuation of, saidtemperature-sensitive switch means for automatically switching the firstfeedwater heater connection from said bleed point of lowest pressure tosaid bleed point of next higher pressure, and vice versa.

2. The combination as claimed in claim 1 further characterized by theprovision of signal means operative to provide an indication when thefeedwater heater connection is connected to the bleed point of highertemperature.

3. The combination as claimed in claim 1 further characterized in thatsaid combination includes a conduit through which combustion-supportingair is carried into said boiler, in that a water-to-air heating elementis located within said conduit, in that a closed-circuit water pipe lineconnects said Water-to-air heating element to a second water coil insaid first feedwater heater, said water pipe lines to said first andsecond coils in said first feedwater heater being free of fluidcommunications with each other and both being in heat exchange relationwith said first feedwater heater.

4. In a steam power generating system; a boiler having an air intakemeans; a turbine driven by steam from said boiler; a condenser connectedto condense the steam from said turbine; conducting means including -astack for exhausting combustion products; a plurality of steam bleedpoints each at a diiferent pressure point on said turbine, at least oneof which is below atmospheric pressure at full load; a plurality offeedwater heaters each having at least one water coil therein; meansconnecting different of said bleed points to different feedwaterheaters, the bleed point of lowest pressure being connected to a firstfeedwater heater and the other bleed points of increasingly higherpressure being connected to second, third, etc. feedwater heaters inthat order; means for pumping the condensed steam from the condenserthrough a first coil in said first feedwater heater and then at pressurealbove atmospheric pressure through a low-level econornizer located inthe path of flow of said combustion products; a preheater arranged totransfer heat from exhaust gases to the incoming air; an air temperingheater located in the path of flow of incoming air ahead of saidpreheater; separate water circulating means comprising a closed circuitsealed from fluid communication to said condensed steam and in heatconducting relation to said air tempering heater and to said firstfeedwater heater for transferring heat from said feedwater heater tosaid air tempering heater; and temperature-sensitive switch meanspositioned to be sensitive to the temperature in said stack forautomatically switching the turbine connection to said first feedwaterheater from said bleed point of lowest pressure to the bleed point ofnext higher pressure and vice versa, said automatic switching meansincluding a first motor-operated valve in the connection from the bleedpoint of lowest pressure in said turbine to said first feedwater heater;a second motor-operated valve in the connection from the bleed point ofnext higher pressure in said turbine to said first feedwater heater,said first and 3,040,537 7 8 second motor-operated valves beingconnected electriized in that said water in said closed circuit is at aprescally to, and operative automatically in response to actuasure aboveatmospheric. tion of, said temperature-sensitive switch means.

R f i t t 5. Apparatus as claimed 1n clalm 4 characterized in e eremes Cted m the file of hls pawn that said first feedwater heater is switchedto a higher- 5 UNITED STATES PATENTS pressure bleed point when the stacktemperature is below 2,788,175 Bourek et a1, Apr. 9, 1957 apredetermined temperature. 2,807,013 Prough et a1 Sept. 17, 1957 6.Apparatus as claimed in claim 4 further character- 2,383,832 Arnow Apr.28, 1959

